The present invention relates to a method of shorting the d-c terminals of converter equipment connected to a d-c source with gate turn-off (GTO) thyristors in the event of short-circuit-related overcurrents. The invention relates also to apparatus for carrying out the method.
In order to provide a background to the present invention, reference is now made to FIG. 1.
FIG. 1 shows, as an example of a GTO-equipped converter equipment, an inverter 1, in the d-c leads 2 and 3 of which, series-connected inductances 4 and 5 and a shunt capacitor 6 indicate that the valve branches participating in carrying the d-c current and their load relief circuits contain capacities and inductances for instance, stray capacitances and inductances. Connected in series with the d-c terminals can also be separate capacitors or inductances, for instance, in order to achieve smoothing of the input d-c voltage or the input d-c current. A short circuit or short to ground in the converter equipment 1 leads to a fault current which increases steeply and suddenly, and which can lead to the destruction of the thyristors. Due to the low current-pulse resistance of gate turn-off thyristors (GTO thyristors), measures must be taken particularly in GTO-equipped converters in order to protect the converter equipment against destruction by shutting it off.
For this purpose, protective inductances 7 and 8 can be used which are arranged in the d-c circuit and are usually provided for limiting the rise of the d-c current during commutations or, if a GTO is used, to protect the latter while the d-c voltage is switched on.
If only chokes 7, 8 are used for protecting the GTO, they must be made unnecessarily large since a GTO in principle has a relatively long turn-off delay time of several microseconds, so that the fault current does not increase within this delay time to values which can no longer be interrupted by the GTO.
For this reason, conventional safety devices, for instance, fuses 9, 10 or fast-action switches are provided as a rule also in GTO-equipped converter equipment, which switch off the converter equipment without switching off the GTO thyristors. These fuses are arranged between the feeding d-c source (indicated by a capacitor 11, for instance, the smoothing capacitor of an intermediate voltage link) and the protective inductances 7 and 8.
If overdesign of the chokes 7 and 8 is dispensed with, the lower current-carrying capacity of switching-off thyristors requires further measures for short circuit protection. For this purpose, a short-circuit path is provided in FIG. 1 between the d-c inputs of the equipment 1 which can be closed via an auxiliary thyristor 12 when an overcurrent due to a short circuit occurs, where the firing command for the auxiliary thyristor 12 is furnished by a monitoring device 13 connected to the d-c voltage source. This auxiliary thyristor 12 then takes over the major part of the fault current in the event of a short circuit.
With this method, a substantially smaller protective inductance is sufficient because the conventional thyristor can be used as the auxiliary thyristor which can only be switched on and has a substantially shorter switching-on delay time and greater current-carrying capacity than a GTO. However, difficulties arise with such a circuit for dividing the current over the original current path leading through the shutting-off thyristors of equipment 1 and the short circuit path leading via the auxiliary thyristor 12. This division of the current over two parallel branches which are practically free of resistance is determined essentially only by the stray inductances and the voltage drops of the semiconductors connected in these branches. Since the auxiliary thyristor short-circuits not only the voltage source 11, but also the d-c circuit in the interior of the equipment 1, only a slightly attenuated d-c current comes about in the equipment 1 which easily can increase for several hundred microseconds to values which can no longer be switched off by the GTO. If within this time, a switch-off command is given to the GTO by the control unit of the equipment 1, the destruction of the former must be expected.
In order to obtain effective short-circuit current protection, it appears to be necessary in the arrangement according to FIG. 1 to improve, on the one hand, the current division through appropriate design of the stray inductances 4, 5 or by separate series inductances, or on the other hand, to suppress by separate intervention into the control all switching-off pulses for the thyristor in the event of a short circuit. This means, on the one hand, an intervention into the control cycle and on the other hand, circuit changes in the design of the equipment 1 and the associated controls.